HMA Charaterization Fatigue Cracking 1

Fatigue Cracking

Senior/GraduateHMA Course

HMA Charaterization Fatigue Cracking 2

Fatigue Cracking

3HMA Charaterization Fatigue Cracking

Mechanisms• Traditionally considered to start at the

bottom and work up to the top

• Crack starts when tensile strain exceeds tensile strength of mix

• When cracks visible on top, full layer cracked

Subgrade

Base

AC Mix t

Longitudinal pavement profile

4HMA Charaterization Fatigue Cracking

Mechanisms• Recent observations of fatigue cracking that starts

from the top at the outside edges of the wheel path

• Tensile stresses due to tire-pavement interactions at surface

Subgrade

Base

AC Mix t

Transverse pavement profile

Transverse pavement profile

5HMA Charaterization Fatigue Cracking

Fatigue Testing

• Most commonly used

• Flexural beam

• Cantilevered beam

• Others

• Diametral fatigue

• Notched beam

6HMA Charaterization Fatigue Cracking

General Terms• Dynamic load

• Load applied using a sinusoidal wave form

• Repeated load

• Load pulse applied then removed

• Rest period between loads

Load

Load

Time

Time

7HMA Charaterization Fatigue Cracking

Flexural Beam Fatigue Testing

• Repeated load preferred to sinusoidal to permit stress relaxation

• Loading can be either constant stress or constant strain

• Failure = 50% loss of stiffness (controlled strain)

8HMA Charaterization Fatigue Cracking

Determining Failure for Constant Strain

0

0.2

0.4

0.6

0.8

1

1.2

100 1,000 10,000 100,000

Numbers of Cycles

Sti

ffn

ess

Rat

io

Failure = 0.5 Stiffness Ratio

Flexural Beam Fatigue TestingLoadingClamps

Clamps for holding beam

Test Results

Strain,

• Results dependent upon how test run

• Constant stress means stiffer asphalt binders perform better

Stress,

Soft

Stiff

Test Results

Strain,

• Results dependent upon how test run

• Constant strain means softer asphalt binders perform better

Stress,

Soft

Stiff

Constant Stress vs. Constant Strain

Subgrade

Base

HMA 100 mm or less

Subgrade

Base

HMA150 mm or more

Strain at bottom of AC layer controls

Stress controls

13HMA Charaterization Fatigue Cracking

Other Fatigue Tests

• Cantilevered beam

• Diametral

• Notched beam

Cantilevered Beam Testing

• Trapezoid beam configuration

• Requires concrete beam be fabricated then sawn

• Fixed at bottom, loaded in a cantilever fashion at top

Diametral Fatigue Testing

• Repeated load (usually)

• Considered less sensitive to mix properties than flexural

16HMA Charaterization Fatigue Cracking

Example of Test Results

0

15,000

30,000

45,000

Cycles to Failure

20C

Test Temperature

Flexural

Trapezoid

Diametral

Reported in SHRP A-404, 1994

17HMA Charaterization Fatigue Cracking

Advanced Fatigue Topics

• Notched-beam test (C* line integral)

• Dissipated Energy

• Models for Predicting Fatigue Life

Notched Beam Testing

• C*-line integral approach

Fixed Movable

19HMA Charaterization Fatigue Cracking

Dissipated Energy

• Dissipated energy is the amount of energy lost for each loading cycle

• Calculated from the changes in stresses and strains for each cycle of testing

20HMA Charaterization Fatigue Cracking

Difficulties

• Research showed that dissipated energy equations are dependent on mix variables and conditions of testing

21HMA Charaterization Fatigue Cracking

Predicting Fatigue from Binder and Mix Properties

• SHRP strain-dependent model

• Asphalt Institute’s DAMA Program

• University of Nottingham

• Shell

22HMA Charaterization Fatigue Cracking

SHRP Strain-Dependent Model

• Low air voids and crushed, rough-textured aggregates• Increase stiffness• Increase fatigue life (constant strain)

• Indicate that asphalt binder property information not sufficient for predicting fatigue life

23HMA Charaterization Fatigue Cracking

QUESTIONS ?